1. Field of the Invention
The present invention relates to a wireless multi-hop network composed of nodes (wireless devices) having a wireless multi-hop function.
2. Description of the Related Art
A wireless sensor network is a network composed of wireless sensor nodes having a sensor, and a base station. In this network, the wireless node collects environmental information (temperature, humidity, material content in the atmosphere, etc.) and transmits the collection result to the base station.
In the wireless sensor network, the wireless multi-hop network technique is used in many cases. The wireless multi-hop network allows the transmission powers of the base station and each node to be reduced and realizes reduction in power consumption by carrying out packet transfer between the base station and the node via one or plural other nodes. This technique is described in, for example, the paper titled “Power-efficient multi-hop routing with a message driven standby control” by Ohkuma et al. in the technical Report of IEICE, NS003-232 (2003-01) (hereinafter, referred to as “Document 1”).
In the wireless multi-hop network, all of the nodes having a wireless sensor must be communicatable with the base station. Therefore, in the wireless multi-hop network, a relay node having a reactive (passive) type routing function is installed at a specified installation point.
In order to form the wireless multi-hop network, the base station makes a node-to-node connectability diagnosis. The node-to-node connectability diagnosis is made as follows. First, the base station transmits a control signal to each node. Then, the node that has received the control signal returns a response to the control signal to the base station. In this manner, the base station grasps the connection relationships between the base station and the node and between nodes based on the response from each node.
However, as shown in FIG. 1, there may appear a group of nodes connectable to the base station (hereinafter, referred to as a “base station group”) and a group of isolated nodes unconnectable to the base station (hereinafter, referred to as an “isolated node group”). As a matter of course, the isolated node is not communicatable with the base station. Therefore, the relay node is installed in order to connect the isolated node to the base station.
Conventionally, the relay nodes are installed on a basis of trial and error. In other words, first, the relay node is temporarily installed at an appropriate installation point and then the base station makes a node-to-node connectability diagnosis. In this manner, the base station confirms whether or not the isolated node is eliminated.
However, in the conventional method, the isolated node is far from being eliminated in many cases. Therefore, a network designer used to change the relay node installation point many times and repeat the same procedure. Therefore, the determination of the relay node installation point requires many man-hours.